The pre ban C Mags used to have a "Y" shaped aluminum support bracket on the rear of the mag at the bottom of the feed tower connecting across to both drums. I don't know if that was true of all pre ban C Mags but the pre bans I have seen have all had that bracket and it was later changed. When looking at the front of the drum there are likely 2 sets of numbers: one on the bottom front of each drum. The number set on the left drum is a stocking number and the number on the right is the one you want to be concerned with as it is the production number/serial number of the drum that Beta Co. will use to determine the date of manufacture. I have seen government mags with an NSN on the top of the left drum. There are threads out there which were set up as repositories to determine pre ban production number/serial numbers, of which I cannot remember any of the links, but like all free information take it with a grain of salt. Best to contact Beta Co. with your number and get something in writing from the horse's mouth